A variety of drilling rigs are used in the formation and preparation of wellbores for production of well fluids or for other well related procedures. Drilling rigs have been designed to selectively drill using coiled tubing and jointed pipe. In one example, a rig includes a base, a mast and a tubing injector used to inject coiled tubing into the wellbore and to withdraw coiled tubing from the wellbore. The rig also includes a top drive mounted to the mast to perform a variety of drilling related operations.
The top drive is functional to provide drill stem rotation for both drilling and tool string make-up. The top drive also supports the rotating pipe load and provides a sealed swivel arrangement for conveying drilling fluid. Top drives can also be used to handle drill string components and to torque connections between drill string components. However, top drives are relatively expensive devices that have high hydraulic power requirements and pose substantial risk to the drilling operation in the event of top drive failure. The potential for top drive failure also is of concern, because top drives tend to be relatively complex devices that are required to provide rotational motion for a variety of tasks.